Second harmonic generation from a homologous series of molecular crystals: impact of supramolecular interactions

Abstract

Optical second harmonic generation (SHG) from molecular crystals is largely controlled by the molecular hyperpolarizability and the relative disposition of the molecules in the crystal lattice. Even though it is known that intermolecular interactions can modify the bulk SHG, strong and direct influence of such effects in crystals has not been widely demonstrated. We present a new family of crystals based on the achiral remote functionalized nonlinear optical (NLO) chromophore, N-(2-aminoethyl)-4-nitroaniline, combined with a homologous series of carboxylic acids, in which the overwhelming impact of supramolecular interactions on the solid-state SHG is clearly manifested. Crystallographic investigations reveal a systematic variation in the superstructure formation of the NLO-phore unit across the series, and the intensities of the SHG from microcrystalline samples are found to correlate with the assembly pattern. Failure of simple additive models in explaining the SHG trend is analyzed using quantum chemical computations of the hyperpolarizability of molecular clusters which establishes the dominant influence of noncovalent intermolecular interactions. The present study illustrates the feasibility of exploiting supramolecular structural features to enhance the NLO response of molecular materials.